In today's telecommunications, digital networks transport large amounts of information. Network services can be, for example, traditional voice phone, facsimile, television, audio and video broadcast, and data transfer. The general public knows new services by catchwords, such as, for example, internet, intranet, teleconferencing, digital television, telebanking, teleshopping, point of sale, ISDN, to name only a few.
Asynchronous Transfer Mode (ATM) has been accepted universally as the transfer mode of choice for Broadband Integrated Services Digital Networks (B-ISDN). Today, ATM-networks are available with a capacity of 155 Mbps (Megabit per second). With the increasing need of information exchange in the global society, the capacity of existing and future networks must be used efficiently. Multiplexers switch different network services to a single network in a such a way that every service is fully maintained and does not disturb other services.
Useful references describing ATM are [1] Clark, Martin P., "ATM Networks, Principles and Use", John Wiley & Sons and also B. G. Teubner Publishers, New York 1996. ISBN 0 471 96701 7; and [2] Onvural, Raif O., Asynchronous Transfer Mode Networks, Performance Issues, 2nd Rev. ed. ISBN 0-89006-804-6, Artech House, Boston, London 1995.
Every kind of service transmitted by ATM-networks has its own requirements, such as, e.g., capacity (bit rate in bit per second, bps) and priority. For example, two persons talk on the phone and exchange at the same time high definition color images between their computers. Both services, the phone service (at e.g., 64 kbps) and the data service (e.g. 1 Megabyte in total), use the same ATM-network. When speaking, the persons will not understand each other when the phone service has a delay of.apprxeq.10 milliseconds or more. They will not recognize a delay of the same magnitude for the data service. The multiplexer periodically switches the phone service to the network so that delays are minimized. The multiplexer switches the data service to the network when the phone service does not require the network. In other words, the phone service has a high priority, and a relatively low bit rate. The data transfer service has a lower priority.
In an ATM multiplexer or switch, data originating from different kind of services (or "channels") is segmented into information packages or so-called cells. Cells are the basic information transfer unit in ATM networks. As used herein, a cell is a fixed number (e.g. 53) of bytes with information to be transmitted ("payload") and control information ("header"). A typical cell structure is described in section 2 of [2].
Scheduling methods play a critical role in the design of ATM multiplexers. The present invention seeks to provide an apparatus and method for transmitting cells in ATM, which allows efficient use of the network.